First of all, it’s not appropriate to use numerals in normal fiction prose. Yes, I know journalism encourages it. Journalism is not fiction. Spell out the numbers: twelve instead of 12. (Numbers for dates are an exception, which will be covered in a later post.)
The big problem here, though, is the lack of hyphens.
You are the twelve-year-old son of a professional assassin.
Not the twelve-year old son or the twelve year-old son. Those three words function together as a compound adjective. (We do not hyphenate them when they’re not functioning as a compound adjective or noun: The twelve-year-old is twelve years old — d’uh.)